This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-284415, filed Oct. 5, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an image processing apparatus and more particularly to an image processing apparatus which accurately separates a shadow and the background, and selectively suppresses a noise component of the background without damaging a contrast of the shadow in a medical image including a shadow potentially dispersed in the background noise, for example, in an X-ray image including a linear shadow such as a vascular shadow and a guide wire and an edge-like shadow.
Catheter therapy under fluoroscopy as a medical technology utilizing X-rays is frequently performed. However, as X-ray fluoroscopy reduces a dose of X-ray compared to that of conventional X-ray radiography so that it may reduce an amount of exposure of the patient, the problem that noises superimposed over an image are so large and a guide wire, a catheter or vascular shadow contrasted is hindered from seeing clearly by the background noise will occur. On the other hand, when performing fluoroscopy by increasing the dose, the background noise becomes relatively small, but it cannot be actually applied because the amount of exposure of the patient and medical concerns is increased. Thus, up to this time, the technologies for reducing noises and enhancing contrasts using image processing have been continued to be studied.
The most generally used image processing technology for reducing the noises is a method of averaging images which are inputted sequentially in time. This technology is already known, and regression filter, which referred to a recursive filter, as its applied technology is also generally used. Addition in time has a noise reducing action for an area without movement, however, with respect to an object moving just like a guide wire inserted into the heart vascular system, it has a drawback to reducing its contrast or resulting in afterglow. Therefore, the weight of averaging is changed per image so that image addition effect can be changed by detecting the movement of an object (See, for example, Japanese Patent Publication No. 2508078, Japanese Patent KOKOKU Publication No. 6-69447, Japanese Patent KOKAI Publication No. 3-198836, Japanese Patent KOKAI Publication No. 6-47035, Japanese Patent KOKAI Publication No. 7-79956, and Japanese Patent KOKAI Publication No. 8-255238).
However, since detection of the movement of an object is essentially performed by a method based on the difference between the present image and the prior image, time changing of pixel density due to the background noise as well as the movement of the object is falsely recognized.
Therefore, when precision for detecting a movement of these such as a catheter whose diameter is minimized as medical treatments progress recently, and a guide wire originally having a small contrast is enhanced, the effect of reducing noises results in its limit. This is because when processing parameter enlarging the effect of noise reduction is set, an image is not clearly seen by generating afterglow instead.
On the other hand, for the purpose of reducing noises and enhancing contrast, linear spatial filter is also used in general. This is a processing for obtaining weighted average of pixels distributed in space and emphasizing contrast by smoothing or by having spatial differential action. However, it has a drawback in damaging a contrast by blurring out a guide wire and a catheter to be linear shadows or edges of organs, and a drawback in noises also being emphasized when contrast is emphasized. Thus, an apparatus for spatially performing pattern recognition of vascular shadows (Japanese Patent KOKAI Publication No. 4-122355), or an apparatus for detecting an edge area of an object, and performing emphasizing contrast in an edge area and smoothing in other areas (Japanese Patent KOKAI Publication No. 60-245084) has been developed.
Since the former requires many averaging circuits and comparator circuits per one pixel unit and it requires a large number of circuits in order to perform a real-time calculation such as fluoroscopy processing and the like, it is not capable of producing the effect of reducing noises. Moreover, since the latter distinguishes an edge area from the background area with only information of one bit, if a false recognition is occurred, a noise which is located adjacently to an area being extremely performed smoothing and falsely recognized, for example, is emphasized and outputted, and it easily damages the quality of an image because the false recognition sensitively appear as an artifact of an output image.
Furthermore, besides those, in Japanese Patent KOKAI Publication No. 9-248920, an image processing method of identifying a linear shadow of an image by determining whether it is a true linear shadow or the background using information of 2 bits is disclosed, whereby resulting in identifying precision being enhanced, but some artifacts are occurred by a false recognition. Moreover, a problem that it cannot effectively perform noise reduction to the linear shadow itself is occurred.
As another method for image processing, a method for determining directivity of a linear shadow or edge, and emphasizing its directional component or separating an area having directional component from other area and separately filtering those areas has been reported. These technologies refers to Directional Adaptive Filters, and some of these technologies are known.
A description of a method for detecting a direction using wavelet transformation with respect to an X-ray image is disclosed in U.S. Pat. No. 5,799,100, or U.S. Pat. No. 5,717,791. Moreover, a method for applying a filter intensified for performing noise reduction at the place where a linear shadow or edge is absent, by analyzing the difference between the original MRI image and the MRI image subjected to 4-direction filtering is disclosed in U.S. Pat. No. 5,602,934. However, for these methods, its algorithms and calculations are too complex to perform real time processing. Specifically, in the case of a method using wavelet transformation, even when performing just only wavelet transformation, it is difficult to perform real time processing. Moreover, a method as described in the specification of U.S. Pat. No. 5,602,934, actually performing a filter having directivity of angles of 4 directions. In this case, however, besides disadvantage in precision when a linear shadow or an edge is present at the angles other than those angles, it has also a problem that the number of filter calculation times and amounts of calculations between images are large. Therefore, more high speed and precision enhancement are required.
Thus, in conventional image processing technologies, it was difficult to suppress not only selectively a noise component of the background, but also in real time without damaging contrast of a linear shadow and an edge-like shadow in a image including a linear shadow or an edge-like shadow. It should be noted that this problem can occur similarly, without limiting to an X-ray image, with respect to the images in general which a random noise is superimposed such as CT image, MRI image and the like.
Accordingly, it is an object of the present invention to provide an image processing apparatus for enhancing the precision of separation between a linear shadow or an edge-like shadow and the background noise, suppress the background noise without damaging the contrasts of the shadows, and in addition to the above, being capable of performing a real time processing.
An image processing apparatus according to the present invention, comprises means for detecting a direction of a linear shadow or an edge-like shadow included in the image and its intensity, and filter means for performing a spatial filtering operation having directivity corresponding to the direction detected by the detecting means and its intensity.
Therefore, it can separate a linear shadow or an edge-like shadow potentially dispersed in the background of the image from the background noise and suppress the background noise without lowering the contrasts of the shadows.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention.
The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.